1. Field of the Invention
The present invention generally relates to printed circuit boards and to processes for making printed circuit boards. More specifically, the invention is related to printed circuit boards comprising conductor paths on an insulating substrate as well as plated through holes being in contact with at least one or none of said conductor paths, as well as to a complete process for making such special circuit boards.
2. Description of the Prior Art
Printed circuits such as described are well known in the fields of electricity and electronics, serving as basic elements in the realisation of electric and electronic circuits and devices for all uses. The flat substrate of insulating material serves simultaneously as a mounting support for the circuit components like resistors, capacitors, semiconductors and integrated circuits, and by the conductor paths which it bears, as connecting means between these components.
The substrate may be a plate made of insulating material such as bakelite, epoxy resin, glass fiber reinforced resins, polyester etc., having a thickness of about 0.1 to 6 mm, for example; it may also be constituted by a more or less thin sheet of soft, elastic or halfrigid insulating plastics material. The conductor paths consist generally of copper or copper alloys and may be tin, tin alloy, silver or gold plated.
The manufacture of printed circuits is likewise known in principle. Generally spoken, one provides a copper clad board wherein the copper layers may be on one or both faces of the board and may have a thickness of, for example, 5 to 100 microns each, other thicknesses being easily applicable for special cases. The copper surface (or surfaces) is then covered by a suitable design which corresponds to the desired connections between the components to be placed. The copper is then removed from all sites where it is not desired, according to said design or pattern, by an technique etching wherein the copper is oxidized and dissolved as a simple or a complex salt. Known etching reagents are, for example, aqueous solutions of iron (III)-chloride or ammonium perchlorate, or persulfate.
Generally, the through holes necessary for the mounting of the components or, as the case may be, for the interconnection between the surface printed circuits, can be drilled at any stage whatsoever of the overall process. These holes are metallized on their walls in order to assure the conductive interconnection between the surface printed circuits or in order to improve the contact surface for the subsequent soldering of components. The metallisation of the hole walls has already been known and can be accomplished by metallization processes well known in the art, for example by electroless plating with optional further galvanic plating.
Landless plated through holes and a process for making same have already been disclosed in U.S. Pat. No. 3,772,101 by L. G. Chumbers and G. J. Rudy, patented from Nov. 13, 1973. This patent describes a method for producing landless plated through holes in printed circuit boards, and the process is conducted in such a manner that the diameter of the holes in the mask used for protecting the copper layer from being etched, is the same as the diameter of the hole in the board. This requirement causes mask registration problems, and there is, as a result, an overhang of plated material on the surface of the finally obtained circuit around its holes. In another embodiment, the drilled board itself is used as a photomask which requires, however, the use of a positive tenting photoresist. This requirement has drawbacks since it will create confusion problems in practice. Furthermore, as the drilled circuit itself is used as a mask, irradiation must be effected from both sides, and the process of reproducing the circuit path pattern must be split up into two masking and four irradiation steps. The making of landless connection holes for components is not disclosed.
Until now, it has generally been considered as necessary that each connection hole must be surrounded by a solderable metallic land in order to guarantee a reliable solder connection between the conductor path and the connecting wire of a component like the integrated circuit pin. This metallic land is normally a preferably tinned "eye"; the metallic surroundings of the hole can also be constituted by the conductor path itself should its width be greater than the hole diameter. The necessity of having a soldering eye around the connecting holes has been based, until now, on defects of the metallic connection between the conductor path and the metallisation of the hole; only an eye or a land was deemed to give the necessary perfect contact. Furthermore, the land was to ascertain a sufficiently great soldering surface in order to keep the component strongly at its place and to guarantee an appropriate electric contact.
The development of electronics toward miniaturizing requires an increasing density of conductor paths and components on circuit boards. The growing integration tends to fewer external components but to more conductor paths and integrated circuits, and it is more and more necessary to use all the space disposible on circuit boards. For example, current technique allows to pass one or two paths between two adjacent land containing connection holes for integrated circuits. The holes which have a diameter of, e.g., 0.8 mm, are distant by the modular pin distance of 0.1 inch or 2.54 mm which leaves a disposible space of about 1.1 mm between the lands of adjacent holes if the lands have a width of not more than 0.3 mm. These values are the optimum of what can be obtained with the current up-to-date technique.
German Offenlegungsschrift No. 1 811 377, laid open on June 18, 1970, alleges as an advantage the possibility to use landless plated through holes as connection holes but does not disclose such a use.
Furthermore, it has been proposed to use so-called multilayer circuits in order to increase the density of printed circuits. This technique comprises the combination of a plurality, at least two, of individual double-layer circuits etched beforehand, by the use of thin isulating sheets pre-impregnated with polymerizable resins such as epoxy resins. These sheets are placed between two printed circuits, and the sandwich composed by two or more circuits, with the intermediate sheets, is then heated under pressure to form a block. As a final step, interconnection through-holes are drilled and their walls metallized as described. However, as this technique is very complicated because of the alignment and connection problem and of the high prices, it is not widely used.
3. Objects of the Invention
It is therefore a first and important object of this invention to provide a printed circuit board of the previously defined nature having through holes which may serve as interconnecting holes for double layer or multilayer circuits as well as in monolayer or any multilayer circuits as connecting posts for components to be soldered thereto.
Another object of the present invention is concerned with printed circuits wherein the density of the conductor paths as well as the overall part density, and the definition of the circuit, expressed as the number of conductors per unity of length has values largely exceeding the highest densities and definitions to be obtained with the techniques now used.
A further and equally significant object of the present invention is to provide a process for making circuit boards with through holes, to be used for interconnection and/or for the direct soldering of components.
Another object of this invention is a process for the preparation of such printed circuits in a particularly simple, economic and rapid manner producing for the first time easily solderable landless contact through holes having excellent electrical contact with at least one conductor path on the circuit or a component.